This invention relates to molten electrolyte fuel cells and more particularly to fuel cells having multiple enclosures which support and isolate individual cells in an array of cells and provide isolated gas chambers for providing fuel or oxidant gases to alternate cells in the array and for removing waste gases.
The fuel cell is a most promising technology for the alternative generation of electricity using low grade fuels. One type of fuel cell, known as the molten carbonate fuel cell as illustrative of the molten electrolyte fuel cell, consists of an anode made of a porous nickel alloy, a cathode made of a porous metal oxide, an electrolyte made of discrete submicron particles of LiAlO.sub.2 and a liquid mixture of lithium and potassium carbonates, and electrically conductive current collector sheets. The electrolyte is interposed between and separates adjacent faces of the anode and cathode, and the current collector sheets are disposed adjacent the opposite faces of the anode and cathode and deliver respectively the positive and negative charges to the extended output circuit.
The fuel cell requires the continuing addition of fuel and oxidant to generate any electrical power. To accommodate this, the anode and cathode each has defined, by means of fabricated passageways or by its highly porous structural formation, a flow matrix for the passage of gases therethrough. A fuel gas (hydrogen and/or carbon monoxide, for example) is passed through the anode flow matrix and an oxidant (air) is passed through the cathode flow matrix. This provides at the anode, that the fuel gas reacts with carbonate ions from the electrolyte to form carbon dioxide and water, giving up electrons to the external circuit; and at the cathode, the carbon dioxide and oxidant react to form carbonate ions, accepting electrons from the external circuit.
Each cell generates about 0.8 of a volt, so that a practical fuel cell device will have many cells (50-1000) assembled in a single stack. The current collector sheets fit between the adjacent cells of a stack, electronically joining these cells together in a series electrical connection, while separating the cathode and anode structures and the reactant gases in the flow matrices thereof. External manifolds direct the fuel and oxidant gases as parallel gas flows to the cells, and collect the waste gas products from the cells. In a practical utilization of the fuel cell device, such as in a power plant, stacks with several hundred fuel cells are contemplated in a typical power supply.
The separator (bipolar current collector) sheets each respectively bears upon the electrolyte structure, "tile", to form a "wet" seal that separates the cell interior from the surroundings. Typically, the compression (i.e., 60 psi) of the stack components provides the primary means for maintaining the gases sealed at the edges, although even momentary release of stack compression can allow gas leakage that might quickly result in total failure of the fuel cell. Mechanical creep of the components over a longer term of operation can also promote gas leakage through the "wet" seals. Electrolyte migration between sandwiched components of a stack held within a common case confinement can reduce the stack efficiency (some cells "dry out" and some cells "flood") and could even lead to cell failure by shorting across the wetted area.
The exact mechanism by which the electrolyte migrates is not clearly understood. Nonetheless, it is known that the cells near the negative end of the stack become flooded while the cells towards the positive end of the stack become depleted or dry of molten electrolyte. Some have suggested that an electrical shunt current through the manifold gasket causes electrolyte migration toward the negative end of the stack. The depletion of electrolyte severely impairs the performance of the affected cells and greatly increases the overall resistance of the stack.
In addition to the above problems, it is also important to provide a relatively equal supply of fuel and oxidant gases to the respective electrodes of each of the plurality of cells.
Accordingly, one object of this invention is to provide a fuel cell construction that is easily fabricated, initially by forming modules or subassembly components which ultimately are assembled with other components to define the finished fuel cell.
A second object of this invention is to provide a fuel cell construction that is reliable in operation and capable of continued operation even with mechanical creep and the resultant reduction in the component thickness over time and/or possible shifting of the fuel cell components relative to one another.
A third object is to provide a molten electrolyte fuel cell in which the cell enclosures for an array of cells are assembled with an array housing to provide a plurality of isolated chambers to provide a source of fuel or oxidant gas and removal of waste gases.
An additional object of this invention is to reduce the compressive force placed on the fuel cell (MCFC) stack during operation. Mechanical creep is a problem with long ter operation of the MCFC. Component thicknesses are reduced with conditions of high temperature (550.degree.-750.degree. C.) and compressive loading (50-100 psi). Without the requirement of compressive loading to maintain the "wet-seals", mechanical creep of all components is significantly reduced or eliminated. Long, stable MCFC operation can be expected.
A specific object of this invention is to provide an electrolyte module formed of electrolyte-support material and a refractory spacer (of alumina, Al.sub.2 O.sub.3, or lithium aluminate, LiAlO.sub.2, for example) arranged as an annulus around the electrolyte-support material, where each is sandwiched between separate sheets of aluminized steel; where the sheets are perforated across their faces within the area defined by the annular spacer; and where the edges of the aluminized sheets proJect peripherially beyond the annular spacer.
Alternatively, another object of this invention is to provide a means for fabricating such an electrolyte module, where in a first step of fabrication, the sheets are sandwiched on the sides of the annular spacer and the same is cured at high temperatures for a short duration (800.degree. C. for 1 hour or less) to fuse or bond the annular spacer and two sheets together. In a next step of fabrication, an electrolyte-support matrix (LiAlO.sub.2) as a slurry is doctored (or extruded) through the perforations into the confines within the annulus and between the two electrolyte-support sheets. This subassembly is cured again at a high temperature and for a short duration (700.degree. C. for 1 to 3 hours) to form a gas-tight seal between the sheets, the electrolyte-support and the alumina spacer.
A further object of this invention is to provide a composite fuel cell design having many similar electrolyte-modules and electrode flow matrices alternately stacked; where the peripheral edges of adjacent pairs of the electrolyte-modules can be welded individually and successively, or simultaneously or as a group to the metallic current collector sheets so as to seal the gases and respective anode or cathode electrode material therewithin.